1. Field of the Invention
This invention relates generally to navigation systems used in aircraft and similar vehicles and, more particularly, to navigation systems that can be useful for an aircraft operating in hostile territory. The navigation system of the present invention is specifically designed to provide an operator with requisite data for decreasing risk from unfriendly activity in a manner permitting more attention to other aspects of the mission.
2. Description of the Related Art
In aircraft assigned missions over hostile terrain, the demands on the flight deck have become increasingly severe. The flight deck must monitor position and flight parameters while pursuing mission objectives. The mission objectives can include penetration of air space protected by hostile antiaircraft ordinance.
The aircraft position is typically monitored by an inertial navigation system. After calibration, the objective is to have the inertial navigation system provide the global coordinates (i.e., latitude and longitude parameters) of the current aircraft position. In the inertial navigation system, small systematic errors are typically present that can cause the current designated position to deviate from the actual position by an amount that increases with time. To remedy these errors, coordinates of known locations over which the aircraft passes are used to provide a correction to the position designated by the inertial navigation system.
More recently, maps of various portions of the globe have become available in which digitized terrain elevations are provided as a function of a grid of (latitude and longitude) locations. The availability of these digitized grid elevation maps has resulted, in; systems that can, based on measurement of the distance between the aircraft the terrain, correlate the position of the aircraft on the grid of the digitized map. These position locating systems generally rely on Kalman filters. For example, the SITAN (Sandia Inertial Terrain-Aided Navigation) system has been described in "Optimal Terrain-Aided Navigation Systems", by L. D. Hostetler, AIAA Guidance and Control Conference, Aug. 7-9, 1978 (SAND78-0874C); "Nonlinear Kalman Filtering Techniques for Terrain-Aided Navigation" by L. D. Hostetler and R. D. Andreas, IEEE Trans. on Automatic Control, Vol. AC-28, No. 3, March 1983, pages 315-323; "SITAN Implementation in the Saint System" by J. R. Fellerhoff, IEEE, 1985, (CH2365-5/86/0000-0089); and "The AFTI/F16 Terrain-Aided Navigation System", by D. D. Boozer, M. K. Lau and J. R. Fellerhoff, Proc. of the IEEE National Aerospace and Electronics Conference, May 20-24, 1985 (0547-3578/85/0000-0351). Other systems have been described in "Performance Analysis of Elevation Map Referenced Navigation Systems" by C. A. Baird, IEEE, 1983, (CH1839-0/83/0000-0064); "A Digital Terrain Correlation System for Tactical Aircraft" by E. P. Bialecke and R. C. Lewis, IEEE, 1983, (CH1839-0/83/0000-0059); and U.S. Pat. No. 4,584,646, issued Apr. 22, 1986, entitled "System for Correlation and Recognition of Terrain Elevation" and invented by L. C. Chan and F. B. Snyder.
In order to enhance the survivability of an aircraft entering hostile air space, several techniques to protect the aircraft and/or to minimize the risk of detection have been developed. Because target acquisition radar system is generally most effective for line of sight targets, a typical procedure has been to keep the aircraft as close to the ground as possible, attempting to interpose terrain features between the aircraft and the radar system. The target acquisition radar system can not then detect the intruding aircraft or can have difficulty separating the intruding aircraft from background noise. To permit an aircraft to fly as close to the ground surface as possible, terrain following (radiation) systems have been developed that, in response to emitted electromagnetic radiation, automatically maintain an aircraft a preselected distance from the ground. These terrain following systems have the disadvantage that the very electromagnetic radiation needed to guide the aircraft increases the detectability. A second technique used to enhance the survivability of an aircraft entering hostile air space is to equip an aircraft with electronic counter measures apparatus. The electronic counter measures apparatus disrupts the signal received by the detecting apparatus of the hostile target acquisition system so that the aircraft is not detected, so that false targets are detected, or so that maintaining a radiation "lock" on the aircraft is not possible. The electronic counter measures apparatus is expensive and is subject to rapid obsolescence as the technological advances provided reduce the effectiveness of currently implemented electronic countermeasures. A similar result can be achieved by an aircraft by releasing (electromagnetic radiation) reflecting chaff. The chaff typically provides a "bright" or dominant background as seen by the target acquisition system, obscuring the image of the aircraft. The aircraft can rapidly leave the area of the chaff becoming once again visible to the target acquisition system. In addition, signal processing of the returned electromagnetic radiation can remove the false background provided by the chaff. The aircraft can also be provided with electronic equipment that permit, when hostile antiaircraft apparatus has "acquired" or has "locked on" the intruding aircraft, the aircraft to engage in defensive maneuvers. Finally, a class of stealth aircraft are becoming operational that are configured to provide a reduced image for reflected electromagnetic radiation (as well as other aircraft detection techniques). These stealth aircraft have elaborate and costly concealment mechanisms that are impractical for reasons of cost to all but a small percentage of aircraft. Non-the-less, stealth aircraft can also benefit from low level and threat avoidance techniques by minimizing the opportunity for visual detection of the aircraft.
A need has therefore been felt for an aircraft navigation system that can not only permit guidance of the aircraft, but can support mission objectives over hostile terrain including support for response to known hostile antiaircraft installations and for covert low level flight. This support can take the form of displays using available data bases which simplify the decision process in the flight of the aircraft.